56 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of the nerve. A nerve can be irritated thirty to forty times, at intervals of 

 three to four minutes, by blows from a weight of 0.485 gram, falling 1 to 20 

 millimeters, the contractions of the muscle, weighted with 30 to 50 grams, 

 varying from minimal (<> from •'> to 4 millimeters in height. Rapidly following 

 light blows or twitches applied to a motor nerve, by the tetanomotorof Heiden- 

 hain or Tigerstedt, excite a -cries of contractions in the corresponding muscles 

 which fuse more or less into a form of continuous contraction, known as 

 tetanus. 



Not only may a nerve be excited by bringing sudden pressure to bear on it, 

 but the sudden removal of weights or a sudden lessening of tension irritates. 1 

 Kiihne lone a<n> called attention to the excitation of sensory fibres of the 

 ulnar nerve of man on the removal of pressure. The cause is probably the 

 irregular return of the semi-fluid parts of the nerve to their normal relations. 



Mechanical applications to nerve and muscle first increase and later lessen 

 and destroy the irritability. Thus pressure gradually applied first increases 

 and later reduces the power to respond to irritants. Stretching a nerve acts in 

 a similar way, for this also is a form of pressure; as Valentin said, the stretch- 

 ing causes the outer sheath of the nerve to compress the myelin, and this in 

 turn to compress the axis-cylinder. Tigerstedt states: 2 "From a tension of 

 up to 20 grams the irritability of the nerve is continually increased, but 

 it lessens as soon as the weight is further increased." 



Surgicallv the stretching of nerves is sometimes employed to destroy their 

 excitability. Slight stretching heightens the excitability and even quite vigor- 

 ous stretching has only a temporary depressing effect unless it be carried to 

 the point of doing positive injury to the axis-cylinder, and of causing degen- 

 eration. As nerves have the power to regenerate, they may recover from even 

 such an injury. 



The irritability of muscles is likewise increased by moderate stretching and 

 destroyed if it be excessive. Thus slight stretching produced by a weight 

 causes a muscle to respond more vigorously to irritants. Similarly tension of 

 the muscles of the leg, produced by slight over-flexion or extension, makes 

 them more irritable to reflex stimuli, as in the case of the knee-jerk and ankle- 

 clonus. Tension must be very marked to permanently alter the irritability of 

 the muscles. 



Effect of Temperature. — Changes in temperature, if sudden and extreme, 

 irritate nerves and muscles. If the nerve or muscle be quickly frozen or 

 plunged into a hot fluid it will be excited and the muscle be seen to contract. 

 The cause of the irritation has been attributed to mechanical or chemical 

 alterations produced by the change of temperature. The ulnar nerve at the 

 elbow is excited if the part be dipped into ice-water and allowed to remain 

 there until the cold has had time to penetrate; as is proved by the fact that in 

 addition to the sensations from the skin, pain is felt which is attributed by the 

 subject of the experiment to the region supplied by the nerve. As the effect 



1 v. I'xhull: Zetockrifl, fur Biologic, 1894, Bd. xxxi. S. 118: 1895, Bd. xxxii. S. 438. 



2 Op. cit., S. 43. 



